Cathode support



Dec. 17, 1957 J. A. DALLoNs CATHODE SUPPORT Filed oct. 18, 1954 I United States Patent O CATHODE SUPPORT John A. Dallons, Inglewood, Calif., assgnor to Hughes Aircraft Company, a corporation of Delaware ApplicationOctober 18, 1954, Serial No. 462,870

6 Claims. (Cl. S15-3.5)

This invention relates to thermionic cathodes and more particularly to a cathode support suitable for use in a traveling-wave tube.

Traveling-wave tubes are electron-stream type tubes which essentially comprise an evacuated envelope, an electron gun disposed at one end of the envelope for producing an electron stream, a collector electrode disposed at the opposite end of the envelope to intercept the stream electrons, and a slow-wave structure disposed about the stream for propagating an electromagnetic wave.

The largest obstacle in the economic exploitation of typical present day traveling-wave tubes is the focusing solenoid which must necessarily be disposed concentrically about the tube envelope to constrain or to confine the electron stream. The initial cost of the solenoid may be larger than the cost of most of the other tube elements.

Furthermore, the large size, weight and power consumption of a solenoid make the eiicient design of a travelingwave tube very diicult.

The power consumption of a solenoid is proportional to its mean diameter. The constraining axial magnetic field which must be maintained determines the radial thickness of the solenoid. It is therefore desirable to reduce the diameter of the traveling-wave tube with which the solenoid is intended to be used. The inside diameter of the solenoid is, of course, limited by the diameter of the envelope of the traveling-wave tube with which it is employed. It has been found useful touse a stacked construction for the tube, i. e. a construction which avoids particularly miniature nuts, bolts and other interconnecting members. The electron gun generally has the largest diameter of any of the traveling-wave tube elements; however, when a stacked construction is employed for the gun assembly, a problem of heat dissipation is encountered. It is desirable that the cathode employed in the gun be heat insulated from the envelope in order to prevent thermal shock breakage of the envelope. It is likewise necessary to support the cathode so that heat may be radiated to the envelope rather than conducted to contiguous tube elements where differential expansions due to heat may cause their breakage.

It is therefore an object of the invention to provide an improved electron gun having a stacked construction.

It is another object of the invention to provide a metallic heat-resistant cathode support.

The cathode support of the present invention essentially comprises a first relatively thin hollow cylinder which has one end disposed about a cylindrical cathode, a ring which is disposed about the opposite end of the first cylinder, and a second thin hollow cylinder which is disposed about the ring. Heat transfer is Ithus substantially limited to radiation, the conductivity of the thin cylinders being relatively low. Although such a cathode support is particularly suitable for the electron gun of a traveling-wave tube and will be described in connection therewith, it is to be understood that the electron gun of the present invention may be used in connection with any electron discharge device requiring an electron gun.

2 ,8 l 7 ,039 Patented Dec. 17, 1957 The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which an embodimet of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose /of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a traveling-wave tube amplifier or -oscillator embodying the present invention and associated circuits;

Fig. 2 is an enlarged sectional view of the electron gun included in the oscillator of Fig. 1;

Fig. 3 is a plan view of a dielectric rod supporting the slow-wave structure of the oscillator of Fig. 1; and

Fig. 4 is an elevational view of the rod shown in Fig. 3.

Referring to Fig. l a traveling-wave tube oscillator is shown comprising an evacuated envelope 10 having an enlarged portion at the left extremity thereof for housing an electron gun 12. Shown disposed concentrically about envelope 10 is an outer metallic cylinder 14 inside of which a crimped or folded metallic member 16 is disposed for conducting heat from two focusing solenoids 18 and 20 which are disposed concentrically about envelope 10 and spaced from each other. The folded member 16 has smooth axially extending surfaces as viewed from its left end to permit the circulation of a fluid coolant about the solenoids 18 and 20. Focusing solenoids 20 and 18 are supplied with current by a source of potential 22 to produce an axial magnetic lield along a predetermined path including the longitudinal axis of the envelope. The field produced by solenoids 18 and 20 may be uniform along the tube envelope and may have a magnitude of 600 to 1200 gauss.

Basically, electron gun 12 comprises a cathode 26 with a lilament 28, focusing electrode 30 and an accelerating anode 32. Filament 28 is heated by means of a source of potential 34, the negative terminal of which is connected to ground. Focusing electrode 30 is maintained at the same potential as cathode 26 because cathode 26 is mounted on and electrically connected to the focusing electrode. Cathode 26 is maintained at ground potential by an appropriate connection to one of the radial pins 36. Accelerating anode 32 is maintained at a potential from 1000 to 3000 volts positive with respect to cathode 26 by means of a connection to another one of the radial pins 36 which is, in turn, connected to the positive terminal of a source of potential 38, the negative terminal of which 1s connected to the positive terminal of potential source 34.

Fig. 2 is an enlarged sectional view of the gun 12 where the cathode support of the invention is shown in more detail. Cathode 26 comprises preferably a molybdenum cylinder 68 which has a sintered tungsten pellet 70 pressed in one end of the cylinder 68 having an aperture in the shape of `a truncated cone. The pellet 70 has a cupshaped recess 71 to provide a hollow cylindrical electron stream. The pellet 70 is made of a sintered mixture of strontium, barium and tungsten. Disposed concentrically and contiguously about cathode cylinder 68 is a thin metallic cylinder 73. The cylinder 73 extends beyond cathode cylinder 68 to the left and a ring 72 is disposed contiguously about cylinder 73 at the left extremity thereof. An outer thin metallic cylinder 74 is in turn disposed about ring 72 and is coextensive with cylinders 73 and 68. Outer cylinder 74 is also disposed about and in contact with focusing electrode 30 which may have a surface of revolution 76 disposed 671/2 degrees with respect to its axis of revolution as is conventional. In order to suppress electron emission from the central recess 71 of pellet 70, a conical metallic element 78 is disposed within the center of the electron stream path and maintained in position by three metallic rods 80 which are preferably spot welded to conical element 78 and focusing electrode 30. In the preferred embodiment of the invention, the base or flat surface of the conical element 78 is polished to reect heat and the conical surface of the element 78 is blackened Vto provide a good radiating surface. Thus, the temperature of element 78 may be maintained as low as possible to reduce the flow of heat to focusing electrode 30 through rods 80. Element 78 then may also be main tained at a temperature low enough to prevent substantial electron emission at the polished surface thereof.

A dielectric cylinder 82 which may be made of quartz is disposed between focusing cylinder 30 and a suitable disc-like extension of accelerating anode 32. A second focusing metallic rod 84 is disposed concentrically within accelerating anode 32 and maintained in that position by a plurality of connecting rods 86 which are welded to r'od 84 and anode 32 in a manner similar to the connecnon of rods 80 to focusingy electrode 30 and conical element 78. Accelerating anode 32 is not maintained in engagement with envelope but is spaced therefrom by three axial metallic ribs 88 which are disposed 120 degrees apart about the anode 32. One end of anode 32 has an annular recess for receiving a second dielectric cylinder 89 to maintain anode 32 in a xed and spaced relationship with envelope 10 whereby gun 12 is aligned with a wave propagating helix 44. Disposed concentrically about and in engagement with envelope 10 is a conductlon matching ferrule 40 which preferably has a length of onequarter of a waveguide wavelength of the wave being generated by the oscillator. A short hollow focusing cylinder 91 is maintained concentrically within ferrule 40 by means of three metal rods 93. Arc-shaped conductive tab 42 is connected to ferrule 40 and to helix 44 at a point 90.

Cylinders 73 and 74 provide a rigid mechanical construction for supporting cathode 26, but they also desirably impede the flow of heat from cathode 26 to focusing electrode 30. It is furthermore desirable to limit the flow o-f heat to envelope 10 to prevent envelope cracking due to differential expansion, particularly in the neighborhood of the metallic anode supporting ribs 88 with which the envelope is in contact. It is moreover desirable to impede the flow of heat to focusing electrode 30 to prevent cracking of the dielectric cylinder 82. Focusing electrode 30 must necessarily be of metal in order to focus the stream of electrons flowing from cathode 26 and 1s preferably made of tantalum to withstand the heat produced by the cathode 26. Cylinders 73 and 74 thus make possible the stacked construction of the gun in that cylinders 73 and 74 may be of tantalum sheet to impede the flow of heat. The cylinders 73 and 74 may be cut and then spot welded about cathode cylinder 68, ring 72 and focusing electrode 30.

It is to be noted that no connecting elements such as bolts are employed throughout the entire construction of the gun. Yet, none of the internal tube elements nor the envelope 10 are liable to fail because of differential thermal expansions or thermal shock. The tube envelope can consequently be made smaller and solenoid power is saved to an appreciable degree. The solenoids may usefully be made even larger than the envelope 10 to allow the passage of air about the outer surfaces of the solenoids. The air may also be employed to cool electron gun 12 and to effectively cool a collector electrode 50 shown in Fig. l at the right of envelope 10.

Tantalum is preferably employed for cylinders 73 and 74, focusing electrode 30, and ring 72 for three reasons. First, the modulus of elasticity of tantalum is high. It will thus not become easily deformed and gun 12 will not become misaligned with helix 44 even at high temperatures. Secondly, like molybdenum and tungsten, tantalum has a low thermal expansion coecient, i. e. it more nearly matches that of quartz than do most other available metals. The last and most important reason for` using tantalum lies in its low thermal conductivity, viz. 0.130 calorie per square centimeter per degree centigrade per second. On the other hand molybdenum has a thermal conductivity of 0.346 calorie/ cm.2/ C./sec. and tungsten about 0.4 calorie/cm.2/C./sec.

Disposed within the tube envelope 10 in Fig. l in the direction of electron ow from electron gun 12 is ferrule 40, matching tab 42, conductive helix 44, a terminating conductive cylinder 46, a conductive wall coating 48 and collector electrode 50. Conductive coating 48 may be obtained simply by brushing a liquid suspension of silver on to envelope 10. The relatively thick silver coating 48 may thus provide a physical limit against which the ferrule 40, terminating cylinder 46 and helix 44 may be pushed. Helix 44 is suitably supported by three dielectric rods 64 which extend at their ends into the ferrule 40 and terminating cylinder 46. Collector 50 is preferably made of a conductive material such as copper to conduct away heat dissipated by the electrons impinging thereon. A concentric cylinder 52 is disposed contiguously to and extending from collector 50 and a plurality of conductive ins 54 positioned radially from cylinder 52 to a second or outer cylinder 56. Fins 54 thus provide an increased surface area.

A rectangular output waveguide 24, which is provided for the oscillator, is disposed transversely about the envelope 10 extending radially upwards in the space between the solenoids 18 and 20. The waveguide 24 has two hollow cylindrical appendages 21 and 23 disposed about the envelope 10 in engagement with the external walls of the waveguide. The appendages 21, 23 are provided with a plurality of longitudinal apertures 25 which also extend through the waveguide walls to permit the circulation of a uid coolant over the entire length of envelope 10. The waveguide and its appendages are particularly designed to prevent energy radiation. To this end, the length of each of the appendages is generally made at least equal to the outside diameter of the envelope 10. The waveguide appendages are fully disclosed and claimed in a copending application Serial No. 462,785, by John A. Dallons, entitled Waveguide Coupling, led concurrently herewith, yand assigned to the assignee of this application.

A forced draft of air may be produced between cylinder 14, sheet 16, and the outer surfaces of solenoids 18 and 20, and also through the appendages 21, 23 of waveguide 24 and between the inner surfaces of solenoids 18, 20 and envelope 10. The air may then ultimately flow about cylinders 56 and 52 and fins 54. The direction of air ow is indicated by arrows 58. Although the direction of air flow is not particularly critical, since the collector end of the envelope 10 will generally be at a higher temperature than that of the gun 12, the direction of air ow from left to right is preferable to provide counter-current heat exchange. Collector 50 is maintained at a potential somewhat above the potential of terminating cylinder 46 and coating 48 by means of an appropriate connection to the positive terminal of source of potential 38. This is done to prevent secondary electrons from reaching the coating 48. All the metallic elements 40, 42, 44, 46 and 48 are maintained at the same potential by means of a connection from a tap -on source of potential 38 to a metallic conductive cylinder 62 on which coating 48 has been partially disposed. Conductive cylinder 62 is connected at its left extremity to the envelope 10 to form an airtight seal. At its right extremity, conductive cylinder 62 is connected to collector 50 by means of a short glass cylinder 100 to provide an airtight connection.

In traveling-wave tubes a type of self-oscillation often occurs as a result ofthe reflection of thermal noise at the collector end of the tube and its subsequent reflection and regeneration at the gun end of the tube. In order to prevent this type of' self-oscillation a lossy coating 66 is disposed Von one end lof the helix supporting rods 64, one of which is shown in Figs. 3 and 4.

In the construction of the oscillator, conductive coating 48 may be easily deposited upon envelope 10 and cylinder 62. The conductive elements 40, 42, 44, 46 and 48 may be assembled together to form a sub-assembly, rods 64 being inserted into cylindrical recesses in the wall of cylinder 40 and being maintained at a predetermined position with respect to cylinder 46 by means of a retaining ring 94 which is disposed about terminating cylinder 46 and in notches 92 of rods 64. Since matching tab 42 is mechanically and electrically connected to cylinder 40 and helix 44 is electrically connected to cylinder 46, the elements 40, 42 44, 46 and 48 may be slid into the envelope against coating 48. Cathode 26 with all interconnected elements including dielectric cylinders S2 and 89 may then be slid into envelope 10 and pushed against cylinder 40. Leads are then welded to the elements of electron gun 12, anode 32 and cathode 26. The gun leads may then be easily welded within the envelope 10 to the radial pins 36.

In the operation of the oscillator of Fig. l, the hollow electron stream is generated by the electron gun 12. The stream is directed through the helix 44 to collector electrode 50. Due t-o thermal noise within the stream a backward wave is induced on the helix flowing toward the electron gun. The backward wave has a phase velocity which is actually in the direction of `electron flow, a mutual interaction of the stream and the wave being thus produced. The wave is thus amplified as it progresses toward the gun 12. An alternating current representing the energy of the wave then iows within the tab 42 and the waveguide 24 is excited. The waveguide 24 then directs the wave energy to a utilization device without incumbent radiation. This is accomplished by making the waveguide appendages 21 and 23 sufficiently long to prevent such radiation. The heat generated at cathode 26 is prevented from being conducted to the envelope 10 by the heat resistant cylinders 73 and 74. The heat generated at the cathode is thus radiated to the envelope. The envelope 10 may be cooled by an axial circulation of air, the longitudinal apertures 25 in waveguide appendages 21 and 23 permitting such circulation.

What is claimed is:

l. An electron gun including a cathode for producing an electron stream, said electron gun comprising a rst thin-walled electrically conductive cylinder, said first cylinder having one end disposed about said cathode and in peripheral contact therewith, a second thinwalled electrically conductive cylinder disposed concentrically about and .spaced from said iirst cylinder, and electrically conductive means for connecting said cylinders at their adjacent ends remote from said cathode whereby continuous electrical connection is provided between said cathode and the end of said second cylinder opposite said conductive means while at the same time providing within a small cylindrical diameter an extended tortuous thermal path therebetween.

2. An electron gun assembly for an electron discharge device having an evacuated envelope, said electron gun assembly comprising a cathode dispo-sed at one end of the envelope for producing an electron stream, a first electrically conductive thin-walled cylinder, said rst cylinder having `one end disposed about said cathode and in peripheral contact therewith, an electrically conductive ring disposed about the opposite end of said rst cylinder and in contact therewith, a second electrically conductive thin-walled cylinder disposed concentrically about said first cylinder and having one end disposed about and in peripheral contact with said ring, a focusing electrode disposed within 4the opposite end of said second cylinder and adjacent said cathode, a dielectric ring disposed about and in contact with said 4focusing electrode, and means connected to said dielectric ring to maintain said gun in 6.,. predetermined spaced relationship with respect to the envelope, said conductive cylinders and ring, said focusing electrode and said dielectric ring providing an extended lthermal path from said cathode to said envelope.

3. In an electron discharge device having an evacuated envelope, an electron gun comprising a cathode disposed at one end of the envelope for producing an electron steam, a first electrically conductive cylinder having a relatively small thickness compared to its diameter, said first cylinder having `one end disposed about said cathode and in peripheral contact therewith, an electrically conductive ring disposed about the opposite end of said first cylinder and in peripheral contact therewith, a second electrically conductive cylinder disposed concentrically about said rst cylinder and having one end disposed about and in peripheral contact with said ring, a focusing electrode disposed concentrically within and in contact with the opposite end of said second cylinder, a dielectric ring disposed about and in contact with said focusing electrode, and an accelerating anode disposed concentrically within and in contact with said dielectric ring, and a number, greater than two, of axially conductive ribs disposed on the exterior of said anode and in engagement with the envelope to support and align said gun in predetermined spaced relationship with respect to the envelope said conductive cylinders and ring, said focusing electrode, said accelerating anode, said dielectric ring and said conductive ribs providing a long thermal path between said cathode and said envelope.

4. An electron gun for a traveling-wave tube including an evacuated envelope, said electron gun comprising a cathode disposed at one end of the envelope for producing an electron stream, a rst electrically conductive cylinder having a relatively small thickness compared to its diameter, said iirst cylinder having one end disposed about and in peripheral contact with said cath-ode, a second electrically conductive cylinder disposed concentrically about said irst cylinder, electrically conductive means connecting said cylinders at their adjacent ends remote from said cathode, a focusing electrode disposed contiguously to and within said second cylinder about said cathode, means for maintaining said focusing electrode in a predetermined spaced relationship with respect to the envelope, a conical focusing element having a at base surface, and means for maintaining said focusing element spaced from and symmetrically on the axis of said cylinders, said focusing element being disposed so that said flat base surface faces said cathode.

5. A traveling-wave tube including an evacuated glass envelope having an enlarged portion and a narrower portion; a slowwave structure disposed about a predetermined electron path and supported within said narrower portion; a collector electrode disposed within said riarrower porti-on opposite said enlarged portion at the end of said path; an electron gun disposed partially within both of said portons; a constraining solenoid disposed concentrically about said glass envelope; said electron gun comprising a hollow, cylindrical accelerating anode disposed within Isaid glass envelope and partially within both of said portions and secured to said narrower porn tion in concentric alignment with said path; a hollow dielectric cylinder disposed about and supported by said anode and extending into said enlarged portion; a hollow cylindrical focusing electrode disposed within and sup-- ported by said dielectric cylinder and extending further into said enlarged portion; a rst elongated, electrically conductive, thin-walled tube extending into said enlarged' portion and beyond said dielectric cylinder, one end of said tube being disposed about in electrical contact with and supported by said focusing electrode; an electrically conductive ring disposed within in electrical contact with and supported by the free end of said tube opposite said' focusing electrode; a second elongated electrically conductive, thin-walled tube disposed substantially entirely' within said first tube in electrical contact with and supported at one end by said conductive ring; and a cylindrical cathode disposed Within in electrical contact with and supported' by the other end of said second conductive tube, said cathode being adapted to emit a stream of electrons focused by said focusing electrode While said conductive tubes provide an extended thermal path from said cathode to said glass envelope.

6. The traveling-wave tube as dened in claim 5 wherein said conductive tubes and ring are fabricated from tantalum.

References Cited in the le of this patent UNITED STATES PATENTS 

